In a radiotelephone system, the portable radiotelephone is configured for radio communication with one or more remote base stations. To save power and increase battery life of the radiotelephone, the radiotelephone system can operate in a slotted paging mode. During slotted paging mode operation, the radiotelephone does not continuously monitor a paging channel. The radiotelephone only monitors the paging channel at predetermined times. During times when the radiotelephone is not monitoring the paging channel, the radiotelephone “sleeps” in a low power mode by disabling certain radiotelephone circuitry, thereby reducing power consumption.
Slotting paging mode is critical to the battery life of portable radiotelephones. Thus, the goal of slotted paging mode operation is to reduce the on time of the radio to a minimum and to disable as much of the radio as possible during sleep periods.
When recovering from a sleep period, or more generally when activating the radiotelephone receiver, the radiotelephone must acquire a radio frequency (RF) link with a base station in the radiotelephone system. Link acquisition and synchronization, as well as other operations such as communication protocols, are defined in an air interface specification. One example of such a specification is the Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS-95, “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-band Spread Spectrum Cellular System” (IS-95). IS-95 defines a direct sequence code division multiple access (DS-CDMA or CDMA) radiotelephone system. Other air interface specifications exist for wireless local loop (WLL) radiotelephone systems, and new air interface specifications are being proposed for advanced wide-band spread spectrum radiotelephone systems (commonly referred to as third generation cellular telephone systems).
Part of the process for a radiotelephone to acquire an RF link with a base station is the radiotelephone finding an appropriate signal that a base station transmits and then synchronizing to the transmitted signal. Synchronization to the transmitted signal is necessary whether the CDMA system is synchronous (e.g. all base stations are synchronized to a common timing reference) or conversely asynchronous (e.g. all base stations are not synchronized to a common timing reference).
For example, in the IS-95 system, synchronization of a radiotelephone with a base station involves the radiotelephone generating local pseudo-random noise (PN) sequence and aligning that PN sequence with the system PN sequence. This is accomplished through the acquisition of a pilot signal transmitted by the base station. The radiotelephone thus contains a sequence generator to generate the PN sequence. The radiotelephone uses a searcher receiver or other mechanism to align the locally generated PN sequence to the PN sequence of the pilot signal transmitted by the base station. Once the pilot signal has been acquired, the radiotelephone acquires a synchronization signal and a paging signal, and the radiotelephone can then correctly demodulate traffic channels and establish full duplex link with the base station.
In slotted paging mode, a radiotelephone periodically checks for messages from base stations. The radiotelephone must decode one or more frames of data every T seconds. For example, in the IS-95 CDMA system, T is calculated by T=1.28*21 seconds, where i is typically set to 0 or 1. In order to extend radiotelephone battery life, part of the circuitry of the radiotelephone is put to sleep between slotted paging messages (e.g. a clock signal is gated off to circuitry being put to sleep).
FIG. 1 is a timing diagram showing how the prior art radiotelephone activates while operating in the slotted paging mode. The PN sequence timing is shown on time axis 400, and the corresponding radiotelephone event is shown on time axis 401.
Time axis 400 shows that a PN roll boundary occurs at time 404. In spread spectrum systems, the PN sequence is usually of a finite length that repeats itself after cycling through the entire sequence; the PN roll boundary marks the starting point of the PN sequence. For example, in the IS-95 system, the PN roll boundary occurs once every 26.66 msec.
Time axis 400 also shows that a frame boundary occurs at time 406. In the IS-95 system, the 80 msec frame boundary occurs once every 80 msec and is aligned with the PN roll boundary. A paging message begins on an 80 msec frame boundary.
Several radiotelephone events must take place before the frame boundary in order for the radiotelephone to demodulate and decode a paging message. Prior to time 402, the prior art radiotelephone is in a sleep state wherein a clock to the receiver modem circuitry is gated off. When the radiotelephone initially entered the sleep state, the microprocessor stored the current PN sequence state. The radiotelephone then remains in the sleep state for a predetermined amount of time, and the microprocessor keeps track of the sleep time to produce an awake state when the radiotelephone is brought out of the sleep mode.
Just prior to time 402, the microprocessor programs the awake state to the receiver modem and reapplies a clock signal to the receiver modem. This awake state represents a best estimation by the microprocessor of the state of the PN sequence when the radiotelephone is brought out of sleep mode. The awake state is thereafter updated real time in an attempt to track the PN sequence.
In prior art spread-spectrum radiotelephones, approximately 90% of the receiver modem circuitry is gated on and enabled at this point. Thus, within the receiver modem unit, clock signals are applied to all of the demodulation branches, the searcher receiver, and associated timing circuitry
At time 402, a WAKE event occurs, and a WAKE pulse loads identical state information into the searcher receiver and the demodulation branches, thereby synchronizing them relative to one another. The searcher unit then searches received signals until a suitable high-energy ray is found. Once a suitable pilot signal is found, the timing of the searcher receiver and all of the demodulation branches are slewed so that their timing matches the PN sequence communicated through the pilot signal. Slewing is a process that involves advancing, delaying, or holding the internally generated PN sequence relative to the received PN sequence. This establishes a timing reference.
In a typical prior art CDMA radiotelephone, the radiotelephone requires approximately 30 msec to acquire a pilot signal and synchronize the searcher receiver and the demodulation branches to the PN sequence; this is marked as time duration 410. Therefore, the WAKE event must occur at least 30 msec before the SLAM event which is to occur at the PN roll boundary at time 404. Since the clocks to the searcher timing unit, the branch timing unit, and the system timing unit have been gated on since the WAKE event, the important timing relationships between them are continually maintained. In addition, during this approximately 30 msec period, approximately 90% of the receiver modem circuitry is enabled, including all non-searcher receiver circuitry within the receiver modem.
Prior art radiotelephone hardware is configured to initiate a SLAM event at the PN roll boundary (e.g. at time 404). A SLAM event is defined as the synchronization of the system timing unit of the radiotelephone receiver modem to the pilot signal PN sequence. The system timing unit controls the timing of the entire radiotelephone receiver modem and directs the operation of the receiver modem. Thus, for a SLAM the microprocessor directs the system timing unit of the receiver modem to synchronize to the timing of the demodulation branches and the searcher receiver.
The SLAM event occurs at time 404. During the 26.6 msec time duration 412, 90% of the receiver modem circuitry is active. At time 406, the demodulator branches generate de-interleaver data and decode the paging message. The receiver modem finishes decoding the paging message at time 408, and the time for this is typically 35 msec, as marked by time duration 414.
In addition to the radiotelephone awaking at predetermined times while operating in a slotted paging mode, the radiotelephone may also be required to wake up to process or respond to other events occurring either synchronously or asynchronously in the radiotelephone. One example of an asynchronous event is a user input, such as the key press of the keypad of the radiotelephone.
Thus it can be seen that the prior art radiotelephone is inefficient for operation in the slotted paging mode. Reduced power consumption of the radiotelephone is a critical performance objective. The reduced power consumption increases radiotelephone battery life, thereby increasing the amount of time that the radiotelephone can operate without having to re-charge the battery. Accordingly, there is a need for a method and apparatus for efficiently and quickly enabling a spread spectrum radiotelephone during operation in slotted paging mode. There is a further need for a method and apparatus for efficiently activating a spread spectrum radiotelephone in response to synchronous and asynchronous events (e.g. initial activation of the radiotelephone).
In addition, the searcher receiver must locate spread spectrum signals having sufficient energy during an initial acquisition mode. Initial acquisition mode is, for example, when a radiotelephone is initially powered on and needs to obtain initial PN sequence timing. In current radiotelephones, users complain of long acquisition times (e.g. a long wait time before a user can use the radiotelephone after turning the radiotelephone on).
Even after the searcher receiver has determined initial PN sequence timing by finding at least one pilot signal ray of suitable signal strength, the searcher receiver must continually search for new signals since multipath conditions, fading conditions, and the location of the radiotelephone constantly change. In fact, a high percentage of dropped calls can occur because the available pilot signals are changing so fast that the searcher receiver of the radiotelephone cannot track the changes.
Finally, new systems requirements might require what is termed mobile assisted hard handoff (MAHHO). This requires the radiotelephone to break a communication link with a first base station, tune to another frequency, look for pilots, return to the original frequency, and then reestablish the communication link with the first base station. The faster this can be done, the more robust the MAHHO will be. Thus, there is a need for a reduction in the time required to acquire a suitable pilot signal of sufficient energy, a minimization of radiotelephone current drain, a reduction in microprocessor required interaction in pilot signal searching and acquisition, and an increase in searcher receiver flexibility for MAHHO.